The present invention relates to an apparatus for determining the roll position of a spinning projectile, missile or the like, with the aid of polarized electromagnetic radiation.
The present invention is applicable to all types of projectiles, missiles or the like which spin in their trajectory and in which the roll position needs to be determined. In particular, the present invention can be used in guided ammunition, that is projectiles which are fired in a conventional manner into a ballistic trajectory towards the target and in which such ammunition receives commands for correction. Because the projectile spins in its trajectory, its roll position must be determined when the command is given. Otherwise, in the absence of roll position-determining devices, errors readily occur when correcting the trajectory.
It is previously known in this art to determine the roll angle in relation to a reference direction in, primarily, missiles with the aid of so-called rate gyros, with subsequent integration.
However, the use of gyros is fraught with a number of technical problems such as drift in the gyro, bearing friction, sensitivity to acceleration and the like. In particular, the sensitivity to acceleration renders the gyro unsuitable for use in a projectile which is discharged from, for example, a gun.
It is also previously known in this art to determine the roll position with the aid of emitted planar polarized radiation, for example in SE 409 902 and SE 407 714. In such instances, use is made of a laser emitter, suitably placed in conjunction with the firing point and aimed at the target. The radiation emitted from the laser emitter is planar polarized either directly through the radiation source of the laser emitter, or the light from the radiation source is caused to pass through a subsequent polarization filter. The plane of polarization of the emitted laser beam will, either through the filter or directly through the radiation source, be established in relation to a reference plane in space. At its trailing end, the projectile is equipped with a receiver which, in planar polarized laser radiation, is provided with polarization filters and is operative to receive the emitted laser radiation from the laser emitter.
Because of the rotation or spin of the projectile, the emitter laser radiation will, after the polarization filter in the receiver, give rise to a varying signal from which the roll position may be determined, albeit with a magnitude of uncertainty of 180.degree., that is half a revolution.
The above-mentioned SE 409 902 discloses one example of how this uncertainty may be eliminated. In this case, it is the missile that emits radiation which is substantially planar polarized, while the receiver is disposed in conjunction with the firing point. In the missile, there is provided a further radiation source which, on a signal from the firing point or at a certain time after discharge of the missile, is separated substantially radially out from the missile. Using measurement equipment, the position of the radiation source in relation to the missile can be determined in the form of an angle and a marking can be realized on the detected signal which, with good accuracy, indicates the roll position of the missile at the moment of separation.
Even though this prior-art apparatus makes determination of the roll position with a relatively high degree of accuracy and without ambiguity, practical problems are involved in providing the missile with a separable radiation source. These problems are further aggravated for projectiles which are discharged conventionally from a gun barrel. Furthermore, the measurement collation apparatus must be such that the position of the radiation source in relation to the missile proper can be determined. Yet a further drawback inherent in such an apparatus is that signal loss will give rise to uncertainty in the roll position determination.